I actually came across an IXB in the middle of the ocean. The weather was REALLY bad, like always when you're about to attack a convoy
I was scanning the sea with my perioscope and suddenly my auto identifier( Yes,I'm a pussy, I know) identified something in the distance as an IXB. I never actually got to see the sub itself (wich is weird considering I indentified it)
Later I saw a sinking freighter